This invention relates to molecular sieve drying systems. In particular, it relates to methods and apparatus for removing moisture from fluid streams and for regenerating or reactivating moisture-laden zeolite particles of the alkali metal aluminosilicate type. Molecular sieves made from natural or synthetic crystalline alkali-metal alumino-silicates of the zeolite type have been found useful for removing selected components from fluid streams. Drying of fluids such as air, petroleum feedstocks or industrial gases has provided a substantial use for molecular sieve.
Gases can be dried to a water content of a few parts per million. In many systems designed to condition feed to cryogenic plants, the gas must be dried to a fraction of a part per million -- low enough to make deriming of heat exchangers a very rare necessity, even when the gas is taken all the way to the liquid phase. This superdrying can be accomplished even when the feed gas is at high temperature because the dewpoints of molecular sieve dehydration are not a function of inlet temperature, and because these unique adsorbents maintain high capacity even when operating at high temperatures. The ability to handle high temperature feed while producing completely dry gas is a unique characteristic of molecular sieve systems. In addition, the performance of molecular sieves is not affected by the degree of saturation of the feed.
The problem of drying large volumes of fluids at a rapid rate is particularly pressing in the operation of petroleum refineries in which large quantities of hydrocarbon fluids are handled daily. The increase in the yield of product which accompanies such reduction in the water content of the charging stock in many instances more than compensates for the cost of drying the charging stock with chemical drying agents. Although the problem of drying hydrocarbon fluids on a continuous basis is a typical large scale application of the present process because of the large volumes of the hydrocarbon streams utilized in the petroleum industry, the process may be used in many fluid streams (whether normally liquid or gaseous) which are essentially non-reactive with the particular desiccant involved in the process. Thus, moist streams such as air nitrogen, carbon monoxide, carbon dioxide, halogenated hydrocarbon chlorobenzene, and others are nonreactive with approppriate inorganic desiccants and may be utilized as feed stocks for molecular sieve drying processes. The sieves are inert to most process fluids and physically stable in normal bed depths even when wet with water.
The desiccant properties of molecular sieves are carried to higher temperatures than those of other adsorbents. Typical capacity is 16.5% at 95.degree. C and 4% even at 230.degree. C. The amount of water adsorbed has little effect on their drying efficiency up to the "break point" (the point where the vapor pressure increases abruptly). Dewpoints below -75.degree. C, even with gases as high as 100.degree. C, may be realized. Molecular sieves dry gases at high superficial velocities even with low relative humidity feed gases. The velocity usually ranges from 10 to 50 m/min with zeolite agents. For drying purposes, smaller-pore-size molecular sieves (3 A) are often employed to reduce coadsorption of other materials.
Synthetic crystalline alkali-metal alumino-silicates of the faujasite type are described in U.S. Pat. Nos. 2,882,243, 2,882,244, incorporated herein by reference.